cURL library -- heap corruption in curl_easy_unescape

Details

libcurl is vulnerable to a case of bad checking of the
input data which may lead to heap corruption.

The function curl_easy_unescape() decodes URL-encoded
strings to raw binary data. URL-encoded octets are
represented with %HH combinations where HH is a two-digit
hexadecimal number. The decoded string is written to an
allocated memory area that the function returns to the
caller.

The function takes a source string and a length
parameter, and if the length provided is 0 the function will
instead use strlen() to figure out how much data to
parse.

The "%HH" parser wrongly only considered the case where a
zero byte would terminate the input. If a length-limited
buffer was passed in which ended with a '%' character which
was followed by two hexadecimal digits outside of the buffer
libcurl was allowed to parse alas without a terminating
zero, libcurl would still parse that sequence as well. The
counter for remaining data to handle would then be decreased
too much and wrap to become a very large integer and the
copying would go on too long and the destination buffer that
is allocated on the heap would get overwritten.

We consider it unlikely that programs allow user-provided
strings unfiltered into this function. Also, only the not
zero-terminated input string use case is affected by this
flaw. Exploiting this flaw for gain is probably possible for
specific circumstances but we consider the general risk for
this to be low.

The curl command line tool is not affected by this
problem as it doesn't use this function.